Circuit for interfacing a processor to a line circuit

ABSTRACT

A line switch for a digital telephone switching system utilizes distributed processing. The line switch comprises one or more switch modules. Each switch module is coupled to a central office switching system via one or more PCM lines which utilize common channel signaling. Each switch module comprises groups of line circuits, each line circuit being coupled to a subscriber line. Three separate distributed processor functions are provided in each line switch module. A first processor is utilized to communicate with the central office switching system and to coordinate the operations within the line switch module. A second processor is provided to control clock generation and rate conversion circuits. Each group of line circuits includes a third processor to control the operations of the line circuits and to gather information from the line circuit. A busing arrangement is provided within the line switch such that both control information and PCM voice data are routed throughout the line switch. 
     A standardized interface circuit is provided which interfaces each line circuit to the third processor and also to the busing arrangement.

BACKGROUND OF THE INVENTION

This invention pertains to telephone switching systems in general, andto an interface circuit for distributed control digital telephoneswitching systems in particular.

The ITT 1210 is an advanced digital switching system manufactured in theUnited States by the International Telephone and Telegraph Corporation.The ITT 1210, which was priorly identified as DSS-1, is described by C.G. Svala, "DSS-1, A Digital Local Switching System With Remote LineSwitches", National Telecommunications Conference, 1977, pp. 39; 5-1 to39: 5-7. As described by C. G. Svala, the ITT 1210 comprises three basicelements, i.e., line switches, a switching network, and a systemcontrol. A line switch interfaces with subscribed lines and to one ormore PCM (pulse code modulation) lines. A line switch may be co-locatedwith the switching network or may be located remoted therefrom. Theswitching network coupled to the line switches by the PCM linescomprises a number of digital switching modules called switch groupseach of which includes a time-space-time network. The system controlincludes a pair of processors with associated program and data memories.The system control directs the operation of the switching network andthe line switches. Communication of control information between thesystem control and a line switch is via a common channel superimposed oneach PCM line. This common channel approach is described in U.S. Pat.No. 4,125,743, issued Nov. 14, 1978 to R. E. Steidl and assigned to acommon assignee.

A large percentage of the equipment costs in a digital switching systemis attributable to the line switches. Also the line switch designdetermines the transmission performance per subscriber line, traffichandling and line signaling functions of the switching system.

It is therefore desirable to provide an improved line switch having alow cost per subscriber line and improved transmission performance.

SUMMARY OF THE INVENTION

In accordance with the principles of the invention, a line switch isprovided which has a low cost per subscriber line and improvedtransmission performance.

In an embodiment of the invention, a line switch comprises one or moreline switch modules. Each module can terminate one or more PCM lines andeach module interlaces PCM data from the PCM lines onto an internal PCMbus structure that operates at a higher data rate and has a greaternumber of channels per frame. Each line circuit of a module has accessto all the channels of the internal PCM bus structure. Further, inaccordance with the invention, a distributed processing controlarrangement is provided wherein subscriber lines are arranged in groupsand the line circuits associated with a group are controlled by a linegroup controller. A line switch processor contained in each line switchmodule is provided for communicating with the system control and fordirecting the operation of the line group controllers.

In accordance with one aspect of the invention, the internal PCM busstructure serves the dual functions of distributing time multiplexed PCMvoice channels data and control data throughout the line switch. Allcontrol data exchanged between the line switch controller and the linegroup controller or any other circuit module is transmitted over theinternal PCM bus structure. Thus, expansion of a module is easilyaccomplished by adding additional line circuit groups or other circuitmodules and extending the PCM bus thereto.

Further, in accordance with the invention each line switch controllerand line group controller includes a multifunctional interface circuitwhich provides an interface between a micro-computer and the internalPCM bus structure.

An illustrative embodiment of the invention is a line switch having oneor more line switch modules each terminating one of more PCM lines whichare connected to the system network. Each line switch module includes aline switch controller which communicates with the system controller viaa common channel signaling arrangement over the PCM lines. The lineswitch controller directs the operation of the various circuits withinthe line switch module. Each PCM line carries frame organized PCM voicechannels with "X" channels per frame. The internal PCM bus structure isarranged such that 2X channels are provided per frame. A rate convertercircuit is provided for interlacing PCM channels from two lines onto thePCM bus structure and for demultiplexing PCM channels on the PCM busstructure to the PCM lines.

More specifically, a receive rate converter comprises memory into whichPCM data from the PCM lines are stored at the PCM line rate. The memoryis read at the PCM bus structure rate in such a fashion that PCM voicechannels data from the two PCM lines is alternately applied to the PCMbus Structure, i.e. first the PCM data for one channel from one PCM lineis read and then the PCM data for one channel of the other PCM lines isread. A transmit rate converter likewise comprises memory into which PCMvoice channel data from the internal PCM bus structure is stored at thePCM bus structure rate. The stored PCM voice channel data is read fromthe memory at the rate of the PCM line such that the stored PCM voicechannels are read and alternately applied to the two PCM lines.

Additionally, the rate converter includes clock generator andphase-locked loop circuitry. The phase-locked loop permits clock signalswhich are generated in a line switch module to be in phase-lockedsynchronization with either of the PCM lines. When two or more linesswitch modules are connected together to form a line switch, thephase-locked loop circuits of each module will utilize the same PCM linefor phase-locking.

Buffer and distributor circuits are interposed in the PCM bus structure.One function the buffer and distributor circuits provide is that ofconnecting the PCM bus structure of the various line switch modules in aline switch. The buffer and distributor circuits include gates whichpermit intra-line switch calls to be connected within the line switchwithout utilizing the system switching network.

Further, in accordance with the principles of this invention, anarrangement is provided for automatic setting of the line circuits gain.

A further aspect of the invention is the provision of another interfacecircuit for interfacing each line circuit to its respective line groupcontroller and to the PCM buses.

BRIEF DESCRIPTION OF THE DRAWINGS

Above-mentioned and other features and objects of this invention willbecome more apparent by reference to the following description taken inconjunction with the accompanying drawing, in which:

FIG. 1 illustrates in block diagram form a digital switching system inaccordance with the principles of the present invention;

FIGS. 2 and 2A illustrate in block diagram form two embodiments of aline switch comprising two line modules that can be employed in thesystem of FIG. 1;

FIGS. 3 and 3A illustrate the traffic handling capability of the lineswitch of FIGS. 2 and 2A, respectively;

FIG. 4 illustrates the format of PCM frames that can be employed in thesystem of FIG. 1;

FIG. 5 illustrates an alternate PCM frame format that can be employed inthe system of FIG. 1;

FIG. 6 illustrates in a general form the distribution of PCM channelswithin the two line modules of FIGS. 2 and 2A;

FIG. 7 illustrates in a general form the control communication pathswithin the line switch of FIG. 2;

FIG. 8 illustrates in block form the interface logic 6 of FIG. 2;

FIG. 9A illustrates the interleaving operation of the RX rate converterportion of interface logic 6 of FIG. 8;

FIGS. 9B, 9C and 9D illustrate in block diagram form the RX rateconverter 20 of FIG. 8;

FIG. 9E illustrates in block diagram form the line switch controller ofFIGS. 2 and 2A;

FIGS. 9F-9N are flow charts illustrating the operation of the lineswitch controller 7 of FIG. 9E;

FIG. 10A illustrates the demultiplexing operation of the TX rateconverter of FIG. 8;

FIGS. 10B and 10C illustrate in block diagram form the TX rate converter21 of FIG. 8;

FIG. 11 illustrates in block diagram form a portion of generator 16a ofFIG. 8;

FIGS. 12A and 12B when arranged as shown in FIG. 12 illustrate in blockdiagram form the PLL and control circuit 18 of FIG. 8;

FIG. 13 illustrates in block diagram form failure detector 1201 of FIG.12A;

FIG. 14 is a block diagram of PLL 1207 of FIG. 12B;

FIG. 15 illustrates in block diagram form the phase detector of FIG. 14;

FIG. 16 is a state diagram for the phase detetor of FIG. 14;

FIG. 17 is a block diagram of divider 1404 of FIG. 14;

FIG. 18 is a block diagram of loop filter 1402 of FIG. 14;

FIG. 19 is a basic loop filter circuit;

FIG. 19A is one embodiment of loop filter 1402 of FIG. 14;

FIG. 20 is a schematic diagram partially in block form of the PLL 1207of FIG. 12B;

FIG. 21 illustrates in block diagram form one portion of the buffer anddistributor 9 of FIGS. 2 and 2A;

FIG. 22 illustrates in block diagram form another portion of the bufferand distributor 9 of FIGS. 2 and 2A;

FIG. 23 is a block diagram illustrating the operation of the buffer anddistributor 9 of FIGS. 2 and 2A;

FIGS. 24A and 24B when arranged as shown in FIG. 24 are timing diagramsin the buffer and distributor 9 of FIG. 22;

FIGS. 25A, 25B and 25C when arranged as shown in FIG. 25 illustrate inblock diagram form a line circuit group in accordance with theprinciples of the present invention;

FIG. 26 illustrates a command word format;

FIGS. 27A and 27B when arranged as shown in FIG. 27 illustrate in blockdiagram form the per line control interface 44 of FIG. 25;

FIG. 28 illustrates the format of a control word directed to the perline control interface 44;

FIG. 29 illustrates the format of a control word directed to a CODEC 43of FIG. 25;

FIGS. 30A and 30 B when arranged as shown in FIG. 30 show in blockdiagram form the gain/balance circuit 41 of FIG. 25B;

FIG. 31 illustrates the format of a control word transmitted over busesPCMLCA, PCMLCB;

FIGS. 32A and 32B when arranged as shown in FIG. 32 illustrate in blockdiagram form the control interface 33 of FIG. 25A;

FIG. 33 illustrates the register status of interface 23 during aself-test operation

FIGS. 34 and 35 are flow charts of the self-test operation of interface33;

FIG. 36 illustrates in block diagram form an automatic gain settingfeature in accordance with the principles of the present invention;

FIGS. 37A, 37B and 37C are flow charts of programs for automatic gainsetting according to FIG. 36;

FIG. 38 is a general block diagram of the detailed drawing of FIG. 14;

FIG. 39 is a general block diagram of another embodiment of FIG. 14;

FIG. 40 is a general block diagram of a third embodiment of FIG. 14;

FIG. 41 is a flow chart of the software architecture of the line switchcontroller 7; and

FIGS. 42A and 42B when arranged as shown in FIG. 42 and FIGS. 42A and42B when arranged as shown in FIG. 43 are flow charts of programs forthe line group processor 34.

DETAILED DESCRIPTION

1. GENERAL (FIGS. 1 and 2)

The stored program controlled switching system of FIG. 1 is a PCM-TDMdigital switching system which includes a switching network 3 controlledby a system controller 5. A more detailed description of the systemcontroller 5 and the switching network 3 is provided in theaforementioned C. G. Svala reference. The system further includes "n"line switches 1 each serving a number of subscriber lines 2. Each lineswitch is coupled to the switching network 3 via one or more PCM lines4. PCM transmission between the line switches 1 and the switchingnetwork 3 takes place over the PCM lines 4. Each PCM line 4 includes, asis well understood, a path for PCM transmission from the switchingnetwork 3 to a line switch 1 and second path for PCM transmission fomline switch 1 to the switching network 3. Each PCM line 4 carriesframe-organized data or speech at any conventional rate. Operation ofthe line switches 1 are controlled by the system controller 5. In oneillustrative embodiment of the invention, a common channel signalingarrangement such as that taught in U.S. Pat. No. 4,125,743 issued Nov.14, 1978 to R. E. Steidl is utilized such that control informationbetween the system controller 5 and the lines switches 1 is transmittedover the PCM lines 4. Typical information is sent by a line switch tothe system controller 5 including the following:

1. Report of line seizures which the line switch has detected; and

2. Report of a fault detected in the line switch.

Typical commands sent by the system control 5 to a line switch includethe following:

1. Request for assignment of subscriber line to a channel or release ofa line from a channel;

2. Request for ringing to a line;

3. Request for maintenance functions to be performed; and

4. Request for party test to identify the party on the line.

FIG. 2 illustrates one of the line switches in greater detail.

Reliability of operation and flexibility in expansion is provided byorganizing the circuits of each line switch in clusters or "securityblocks" of circuits that function in clusters or "security blocks" ofcircuits that function as a unit. For any single fault, only the groupof circuits within the security block containing the fault may be lostto service. Thus, a single fault will not cause a loss of service to theentire line switch. As will be evident to those skilled in the art, thevarious circuits in the illustrative line switch described herein may beorganized without the security block arrangement shown or the size ofthe various security blocks may be changed.

In accordance with the security block organization of the line switch,the illustrative line switch is organized as two identical line switchmodules. Each line switch module is self contained to the extent that itcan operate as a line switch for a number of subscriber lines.

A further aspect of the novel line switch is that distributed processingis advantageously utilized. Three separate distributed processorfunctions are provided in each line switch module. First, one processoris utilized to communicate with the system controller 5 of FIG. 1 and tocoordinate the operations within the line switch module. A secondprocessor is provided within a line switch module to control clockgeneration and rate conversion circuits. A third processor is providedfor a group of line circuits to control the operations of the linecircuits and to gather information from the line circuits.

A further aspect of the line switch architecture is that an internalbusing arrangement is provided wherein both conrol and PCM data arerouted throughout the line switch. With this arrangement, a standardizedinterface between each processor and the PCM bus may be provided.

In addition to having the capability for establishing connectionsbetween subscriber lines connected to the line switch and the switchingnetwork 3, the present line switch has the capability of establishingintra-line switch connections without the switching network 3.

In accordance with the aforementioned security block organization theline switch 1 is organized as two identical line switch modules 1a, 1b.Each of the line switch modules 1a, 1b is self-contained to the extentthat it can operate as a line switch and is connectable to up to 160suscriber lines or loops. The 160 lines associated with a module arearranged as two clusters of lines. Each cluster is in turn divided into10 groups of lines, each group includes 8 lines and in the illustrativeembodiment each group of 8 lines has its associated line circuitsarranged on one circuit card. It should be noted, that in otherembodiments, a group may contain more or less lines and the lines may bearranged in one or more clusters.

Each line switch module 1a, 1b is coupled to the switching network 3 ofFIG. 1 via two PCM lines PCMB; bit clock lines CLKA, CLKB; and frameclock lines FSA, FSB. The PCM lines PCMA, PCMB operate at a 1.544 mbs.rate carrying 24 voice channels in a frame in the North American networkor at a 2.048 mbs. rate carrying 30 voice channels in a frame inEuropean and many foreign networks. Although two PCM lines are shown,the line switch module may be connected to only one PCM line.

The frame clock lines FSA, FSB each provide a 4 KHz signal from theswitching network 3 of FIG. 1 and each is used to identify whichincoming bit is a common channel signaling bit. The common channelsignaling bits occur in the 193rd bit position of every other frame. Theintermediate 193rd bits are synchronization bits and are used todetermine the A and B signaling frames.

The bit clock lines CLKA, CLKB each provide a 1.544 mHz clock.

The interface logic 6 operates to derive various clocking signals foruse in the line switch module. More significantly, interface logic 6multiplexes and interlaces the PCM channels from the two PCM lines PCMA,PCMB onto bus PCM4 and demultiplexes and separates PCM channels from busPCM4 to the PCM lines PCMA, PCMB. More specifically, the bus PCM4operates at a 4.096 mbs. rate. Data transmitted over bus PCM4 isarranged in frames of 512 bits, 480 of which are used for PCM. If thelines PCMA, PCMB operate at a 1.544 mbs. rate with 24 channels perframe, the channels on the lines PCMA, PCMB will be interlaced and rateconverted to provide 48 channels each 10 bits wide at a 4.096 mbs rateon bus PCM4. If the lines PCMA, PCMB operate at a 2.048 mbs rate with 30channels per frame, they will be interlaced and rate converted toprovide 60 channels each 8 bits wide at a 4.096 mbs. rate on bus PCM4.The remaining thirty two bits of each 512 bit frame on PCM4 are used forinternal control of the line switch module.

The line switch controller 7 receives common channel commands from thesystem controller 5 of FIG. 1 over the PCM lines PCMA, PCMB and via theinterface logic 6. The line switch controller 7 communicates with andcontrols the various circuits of the line switch module via bus PCM4 byutilizing the aforementioned 32 bits.

Buffer and distributor circuits 9 are connected to the bus PCM4 andserve to buffer PCM signals between the bus PCM4 and the line cards 11,ring circuits 13, and other circuits. Two buffer and distributorcircuits 9 in each line switch module are provided to partition the PCMbuses into segments, i.e., security blocks, which are isolated from eachother to prevent propagation of faults in the line switch and the impactof faults in a group of lines upon all other lines in the line switch.Both buffer and distributor circuits in a line switch module containidentical circuitry for buffering and distributing PCM signals and eachis connected to one of two clusters of line circuits. Additionally, thebuffer and distributor circuits 9 provide buffering to the PCM buses inthe other line switch module of the line switch. Thus, each line switchcontroller 7 has access to all the line circuits in both line switchmodules of a line switch and PCM signals may be coupled to all circuitsin a line switch. The buffer and distributor circuits 9 includes gatingcircuits which provide for intra-line switch calls between any two linecircuits within the line switch without the need to route the callsthrough the switching network 3 of FIG. 1. Additionally, one of thebuffer and distributors 9 of a line switch module includes circuitry forderiving various clocking signals from the various clock lines CLKA,CLKB, FSA and FSB.

Each buffer and distributor 9 is coupled to all the line and othercircuits in a cluster by two PCM buses PCMLCA, PCMLCB each of which runsat a 4.096 mHz rate. One of the PCM buses, e.g. PCMLCA of line switchmodule la, is derived directly from the bus PCM4 in the same line switchmodule. The other of the PCM buses, e.g. PCMLCB of line switch module1b, is derived from the bus PCM4 of the other module. In the NorthAmerican Network each bus PCMLCA, PCMLCB will have 48 PCM channels at a4.096 mHz rate. Thus each line circuit will have access to 96 PCMchannels in the line switch.

As pointed out hereinabove, each group of 8 line circuits is arranged ona single circuit card. Each of group circuits is controlled by amicroprocessor which is in turn controlled by the line switch controller7 via the 32 control bits available during each frame. By providing amicroprocessor on each line card the work load on the line switchcontroller 7 is reduced, flexibility of application is obtained, andmaintenance functions may be more easily provided.

Ring cards or circuits 13 are provided in the line modules. The ringcards 13 also include microprocessors which reside on the ring cards.The ring cards contain their own ring frequency generator and a ringcard is controlled by the line switch controller 7 via the 32 controlbits available during each frame. Timing of the ringing is done via themicroprocessors which reside on each line card and on each ring card.

Each line module also includes a power converter 12.

2.0 TRAFFIC HANDLING CAPACITY (FIG. 3)

The line switch shown in FIG. 2 can as noted above be configured suchthat each line switch module 1a, 1b is connected to the switchingnetwork 3 of FIG. 1 by one or more PCM lines PCMA, PCMB. Also, the lineswitch may be operated as either a 24 or 32 channel system. FIG. 3illustrates the traffic handling capability of the line switch of FIG. 2for different numbers of PCM lines in 24 or 34 channel systems.

3.0 PCM BUSES (FIGS. 4, 5, 6 and 7)

Each of the PCM buses PCMLCA, PCMLCB and PCM4 in the line switch of FIG.2 carries information in frames of 125 microsec. in duration. One framecomprises 512 bits. Two alternate arrangements for formatting the 512bit frame are shown in FIGS. 4 and 5 which may be found on the samesheet as FIG. 1. If the line switch is to be used in a 24 channelsystem, the frmaes will be arranged as shown in FIG. 4. Forty-eightchannels of 10 bits each and 32 additional bits which are utilized ascontrol bits for control of internal line switch functions are provided.By providing 10 bit channels, the line switch may be readily utilized inintegrated digital switches proposed for the future wherein each channelwill be arranged as shown in FIG. 5. FIG. 5 shows each frame arranged assixty 8-bit channels and 32 control bits. Other formats may easily beutilized for use in other systems.

FIG. 6 illustrates the distribution of PCM channels in the line switchof FIG. 2 when used in a 24 channel system. To further simplify anunderstanding of the PCM channel distribution, only one cluster of linegroups 11 and only one buffer and distributor 9 is shown for each lineswitch module 1a, 1b. The format of FIG. 4 is utilized. In each lineswitch module 1a, 1b two 24 channel, 1.544 mHz PCM lines PCMA, PCMB areconnected to the interface logic 6. The interface logic 6 provides rateconversion and interlacing between the PCM lines PCMA, PCMB and the 48channel, 4.096 mHz bus PCM4. The buffer and distributor 9 of each linemodule connects bus PCM4 to a 48 channel 4.096 mHz bus PCMLCA within thesame module and via an inter highway bus 1H1 or 1H2 to a 48 channel,4.096 mHz bus PCMLCB in the other module.

Therefore, each line circuit has access to a total of ninety-six 10 bitchannels since PCMLCA and PCMLCB are each 48 channel buses. Further,with this configuration each line circuit has acces to the two 24channel PCM lines PCMA, PCMB in the same module and also to the two 24channel PCM lines PCMA, PCMB in the other module.

As pointed out hereinabove, the line switches 1 are controlled by thesystem controller 5 of FIG. 1. The line switch controller 6, in turncontrol the operation of the line circuits 11 and other circuits withina line switch. Turning now to FIG. 7, the line switch controller 7communicates with the system controller 5 via a common channel signalingarrangement over the PCM lines PCMA, PCMB as described in theaforementioned R. E. Seidl patent. If it is assumed that the PCM linesoperate at a 1.544 mHz rate and one common channel signaling bit isprovided in every other frame, then the common signaling channeloperates at a 4kbs rate, i.e. 1 bit/2 frames×8000 frame/sec=4kbs. Thebuses PCM4, PCMLCA, PCMLCB are used to transmit 32 bits of control dataper frame, each frame being 125 microsec. in duration. The buses PCM4,PCMLCA, PCMLCB operate as 256 kbs control data channels, i.e. 32bits/frame×8000 frames/sec=256 kbs. A line switch control 7 thusprovides a common 256 kbs control channel to each line circuit 11 orother circuit (which may be the ring circuit 13 or power convertercircuit 12 of FIG. 2) via buses PCM4 and PCMLCA in a line switch moduleand additionally provides via bus lH1 or lH2 the same 256 kbs controldata channel to other line switch modules in a line switch. Furthermore,bus lH2 or lH1 via bus PCMLCB provides a 256 kbs control channel fromthe other line switch modules in a line switch to the line circuits ofthe line switch module shown in FIG. 7.

4.0 INTERFACE LOGIC 6 (FIG. 8)

4.1 General

The interface logic 6 shown in block diagram form of FIG. 8 providesrate conversion and interlacing/demultiplexing of data between the twoPCM lines PCMA, PCMB and the bus PCM4. Bus PCM4 and other PCM buses inthe line switch are clocked at a 4.096 mHz bit rate. These mustinterface to the PCM lines PCMA, PCMB which clock PCM at a 1.544 mHz bit(or a 2.048 mHz bit rate in other applications). The interface logic 6provides for the line switch internal clocks to be synchronized to thePCM line clocks CLKA, CLKB at frame boundaries such that no PCM bits arelost during rate conversion. This is accomplished by phase locking theinternal clock to a PCM line clock by means of phase-locked loop (PLL)circuitry.

The rate converter portion of interface logic 6 includes memory elementsfor the PCM bit streams. The PCM data from bus PCM4 are written into amemory in the rate converter as the data for PCM lines PCMA, PCMB areread from the memory. Likewise, PCM data from lines PCMA, PCMB arewritten into a memory as the data for bus PCM4 is read from the memory.

Each of the two PCM lines PCMA, PCMB is connected to one of twocorresponding identical signaling frame and frame clock generatorcircuits 16a, 16b. Each generator 16a, 16b derives an 8 khz clock signalon leads 17a, 17b, respectively, from the bit clock signal received viathe respective bit clock leads CLKA and CLKB and determines the presenceof a signaling frame, and generates a signaling frame signal. The 8 kHzclock is applied to a phase-loop and control circuit 18 and is suppliedto a corresponding phase-locked loop and control circuit 18 in anotherinterface 6 via leads FS8A or FS8B. The phase-locked loop and controlcircuit 18 generates a 4.096 mHz clock which is phase-locked to aselected 8 kHz clock on lead 17a, 17b or on the leads FS8A, FS8B whichcome from an interface logic circuit 6 of another line switch module.Thus, the phase-locked loop 18 of a line switch module may be locked toany one of four 8kHz clocks. The 4.096 mHz clock is provided on leadCLK4 and also to a clock generator circuit 19. The generator 19 providesa frame signal at an 8 kHz rate and clock signals at 4.096 mHz rate forthe transmit (TX) and receive (RX) rate converter circuits 20 and 21respectively.

The PLL and Control Circuit 18 selects which one of the four 8 kHzclocks on lead 17a, 17b, FS8A and FS8B is to be used as a master clockof the line switch. The phase-locked loop and control circuit 18contains failure detection circuitry for all the 8 kHz clocks. When theabsence of a clock pulse occurs the failure detector will insert a pulseto maintain the oscillation frequency of PLL, and another one of theremaining working 8 kHz clock will be selected as a master.

Additionally, there is cooperation between the PLL and control circuits18 in the modules of a line switch such that all will operate to selectthe same 8 kHz derived clock as a master. This assures that the PLL's ofa line switch modules are synchronized to the same clock source.

Each PLL further includes self-check circuitry such that, if a PLLfails, the PLL will automatically disconnect iself from the 4.096 mHzline CLK4 and connect the line CLK4 to the output of the PLL in theother line switch module.

4.2 RX RATE CONVERTER (FIGS. 8, A-D)

Rate conversion and interlacing/demultiplexing are provided primarily bythe TX rate converter 20 and the RX rate converter 21. The bus PCM4includes a transmit path 8a and a receive path 8b. Likewise, the PCMlines PCMA, PCMB have both transmit and receive paths.

Incoming PCM signals from the switching network 3 over lines PCMA, PCMBare applied to the RX rate converter 21. The RX rate converter 21operates as a FIFO type memory. Information received over PCM linesPCMA, PCMB is stored at the rate of the PCM lines, i.e. 1.544 mHz forthe North American Network or 2.048 mHz for the other systems. Morespecifically for the North American Network data from PCM line PCMA isclocked into RX rate converter 21 at a 1.544 mbs rate as determined bythe 1.544 mHz PCM clock line CLKA of PCM line PCMA. Likewise, data fromPCM line PCMA. Likewise, data from PCM line PCMB is clocked into the RXrate converter 21 as determined by CLKB. The data stored in the RX ratecoverter 21 is read at a 4.096 mbs rate as determined by the 4.096 mHzoutput of clock generator 19. The 1.544 mHz clocks signals received overCLKA, CLKB are not necessarily synchronized. However, the 4.096 mHzclock signals are phase locked to the clock signals of one of the PCMlines connected to a line switch.

Data from the two PCM lines PCMA, PCMB is alternately read from the RXrate converter and applied to the receive portion of bus PCM4, i.e.,path 8b. FIG. 9A illustrates the interleaving of channels from the PCMlines PCMA, PCMB onto the bus PCM4.

The RX rate converter 20 is shown in greater detail in FIG. 9B, 9C and9D. FIG. 9B illustrates the interconnection of the circuits of FIGS. 9Cand 9D. The circuitry of box 951 from PCM line PCMA is duplicated in box952 for PCM line PCMB and is shown in detail in FIG. 9D. FIG. 9C showsbox 990 in detail. Turning now to FIGS. 9C and 9D, serial PCM data isreceived from PCM line PCMA. Alternately received bits are stored inbuffer flip-flops 902 and 903 by flip-flop 901. After every other bit isstored in the flip-flops 902, 903, the write address counter 904 isadvanced by one count and the two bits stored in flip-flops 902, 903 arestored in the memory 905. The flip-flops 902, 903 act as a two bitserial to parallel converter and thus form two bit words for storing inthe memory 905. Memory 905 is organized as sixteen 2 bit words and is arandom access memory. Memory 905 includes four address bit inputs andread/write input. A read write control circuit comprising flip-flops907, 908 and gates 920, 921 determines whether a read or write operationis to occur. Address selector 909 is used to select either a writeaddress from counter 904 or a read address from counter 910.

When the data stored in flip-flops 902, 903 is to be stored in memory905, the selector 909 selects the counter 904 as the source for thememory address and flip-flop 908 applies a write signal to the memory905. When data is to be read from memory 905, selector 909 selectscounter 910 as the source for the memory address and flip-flop 908provides a read control signal to the memory 905. Data read memory 905is read as two bit words which are alternately stored in the two bitlatches 911, 912. The flip-flop 932 and gates 933, 934, 935, 936 and 937control the loading of data read from memory 905 into the latches 911,912. A selected 913 under control of flip-flop 915 alternately gates theoutputs of the latches 911 912 onto the two bit lines 914. Tri-statebuffer gates 916, 917 controlled by gates 938, 939 alternately connectone of the lines 914 to the single line 961. The flip-flop 931 is usedto select either the outputs from box 951 or 952 to be applied to theline 961. Flip-flop 931 changes its output state once for each voicechannel on PCM4.

In effect, latches 911, 912; selector 913 and gates 916, 917 operate asa parallel to serial converter and flip-flop 931 operates to selecteither PCMA or PCMB as the source of data on PCM4. The read addresscounter 910 is driven by signal MCLK which as will be described belowdefines the format of PCM4. Pulses are provided by MCLK which correspondto the PCM voice bits on the bus PCM4.

The gates 938, 940, 941, 942, flip-flop 939, decade counter 943 and 6bit counter 944 operate to control the tri-state buffer gate 945. Gate945 is enabled when PCM data from either PCM line PCMA or PCMB is to betransmitted over PCM4. In accordance with the format for PCM4 shown inFIG. 4, gate 945 is enabled for the 8 PCM sample bit positions of each10 bit channel and is disabled for the remaining two bits. Also, gate945 is disabled for the 32 control bit positions in each frame.

4.3 TX RATE CONVERTER (FIGS. 8, 10A, B and C)

The TX rate converter 20 of FIG. 8 mirrors the operation of the RX rateconverter 21. Specifically, the channels of data on the transmit portion8a of bus PCM4 are stored in a FIFO memory of the TX rate converter 20.Information is received at a 4.096 mbs rate and stored under control ofthe 4.096 mHz clock signals of clock generator 19. Alternate channels ofdata stored by the TX rate converter are forwarded to the PCM linesPCMA, PCMB at a 1.544 mbs rate as determined by the respective clocksignals CLKA, CLKB or the PCM lines. This demultiplexing operation isshown in FIG. A.

The TX rate converter 21 of FIG. 8 is shown in detail in FIGS. 10B and10C. The circuitry in box 1051 for PCM line PCMA is duplicated in box1052 for PCM lines PCMB. Boxes 1051 and 1052 are shown in detail in FIG.10C.

Serial PCM data is received over bus PCM4. Alternately received PCMsample bits are stored in the two buffer flip-flops 1002, 1003.Flip-flop 1004 which is clocked by the signal MCLK selects which of theflip-flops 1002 or 1003 into which a PCM bit is to be stored. Theflip-flop 1005, and gates 1006, 1007 are used to synchronize theoperation of flip-flop 1004 each frame. After every other bit is storedin the flip-flops 1002, 1003, the two stored bits are written intomemory 1008. Memory 1008 is a random access memory organized as 16 wordsof two bits each. Memory write addresses are generated by the counter1009. Counter 1009 is a 5 bit binary counter which is clocked at onehalf the rate of the MCLK signals by flip-flop 1004. The C output of thecounter 1009 is used to control gate 1010 and through an inverter 1011controls if the corresponding memory on box 1052 is to be written orread. Decode counter 1012 and gates 1013, 1014 control whether data isto be stored in or read from memory 1008 and also counts the number ofbits (10) in each channel on bus PCM4.

When counter 1012 contains a count of 0 to 1, a memory write operationis inhibited. Address selector 1015 is used to select the address inputto memory 1008. Specifically, the selector 1015 selects output fromcounter 1009 for memory write address and counter 1016 for a memory readaddress. Counter 1016 is a 5 bit binary couner and is clocked by the PCMline clock CLKA. When data is read from memory 1008, the two bit wordsare alternately stored in the two bit latches 1017, 1018. Flip-flops1019, 1020 and gates 1021, 1022, 1024, 1025 provide control for gatingthe data into the latches 1017, 1018. Gate 1023 in combination with the"C" output of counter 1016 provides for alternately selecting thelatches 1017, 1018 for storing the read data. The selector circuit 1026under control of flip-flop 1027 alternately gates the contents oflatches 1017 and 1018 on line 1028. Tri-state buffer gates 1029 and 1030controlled by gates 1031, 1032, 1033 are alternately enabled toalternately connect the two outputs of the selector 1026 to PCM linePCMA. Latches 1017, 1018, selector 1026 and gates 1029, 1030 operate asa parallel to serial converter.

The flip-flop 1055 clears the counters 1016 whenever the framing bitposition during a frame on the PCM line PCMA occurs.

Tri-state buffer gate 1036 is enabled during the framing bit position topermit common channel signaling or framing information to be gated ontoline PCMA.

The above described interlacing and demultiplexing operations arepremised upon the use of 24 channel 1.544 mHz PCM lines PCMA, PCMB andthe bux PCM4 having the format of FIG. 4. If, however, the PCM linesPCMA, PCMB are 30 channel 2.048 mHz lines then the PCM clock lines CLKA,CLKB would operate at 2.048 mHz and the format of FIG. 5 would be usedfor bus PCM4.

4.4 PHASE-LOCKED LOOP AND CLOCK CIRCUIT (FIGS. 2,8,11-20)

The rate converters require that the clock frequency of bus PCM4, i.e.,be exactly 512/193 times the clock frequency of the PCM lines PCMA,PCMB. The phase difference between the two clocks must be controlledsuch that it falls within the limits required by the rate converterhardware.

The maximum phase difference between the internal line switch clock andthe PCM line clocks that is tolerable to the rate converter is limitedonly by the amount of memory storage in the rate converter. However, alarge tolerance for phase variation requires a larger amount of memorystorage and causes a larger nominal delay in the rate converter.

The rate of converter inherently causes 20 microseconds of nominal delayinterfacing the 4.096 mHz PCM to 1.544 mHz PCM. Additional delay isdesigned into the rate converter to tolerate the phase variation in theinternal line switch clock relative to the PCM line clocks ClKA, CLKB.To design the rate converter for minimum delay requires the phasevariance of the clocks be tightly controlled.

The rate converter requires a specific relationship between the twoclocks. This relationship is a defined sequence which repeats everyframe (125 microseconds). At frame boundaries, (Bit 193 of the PCM lineclocks) the rising edges of both clocks are synchronous when they are inphase. Any deviation is considered a phase error of the line switchclock.

In the illustrated embodiment of FIG. 2 two 4.096 mHz buses PCM4 aredriven by the same clock, i.e. one bus in each of the line switch moduls1a, 1b. Up to four external PCM lines, PCMA, PCMB in both line switchmodules, whose phase relative to one another can vary, will interface tothe two buses PCM4. Since the 4.096 mHz clock will be phase-locked to anarbitrarily chosen PCM line, the rate converter must be capable oftolerating the phase difference relative to other PCM lines. Also, thefailure of any one PCM line must not affect service on the other PCMlines. An extension of this philosophy requires that any single failurein the PLL circuitry not cause the loss of all four PCM lines. Insummary, the

PLL must have a phase error whose variance is minimized relative to thePCM lines;

The PLL must be capable of using any one of the PCM lines as a referenceso that a failure of any PCM line doesn't affect service on the otherPCM lines; and

Two PLL circuits must exist that are independently selectable to ensurethat a single failure in the PLL does not affect the PCM clock CLK4.

The signaling frame and frame clock generators 16a, 16b of FIG. 8includes a divider circuit to derive an 8 kHz signal from the 1.544 mHzPCM line clock and the signaling frame clock. The divider circuit forgenerator 16a is shown in detail in FIG. 11. The circuit includes gate1101 connected to the input of an eight bit counter 1102. Decoder gate1103 coupled to the eight bit counter provides an output to the "D"input of flip-flop 1104. Flip-flop 1104 has its clock input CLKconnected to CLKA and its Q output provides an 8 kHz framing signalFS8A. The outputs FS8A and FS8B of generators 16a, 16b are provided asinputs to the PLL and control circuit 18 of FIG. 8. Additionally theseFS8A and FS8B outputs are provided to the other line switch module on aline switch. Similarly, FS8A and FS8B outputs from the other line switchmodule are provided as inputs to the PLL and control circuit 18 of FIG.8. Thus, the PLL and control circuit has clock inputs derived from eachof the four PCM lines connected to a line switch, i.e., the signalsderived from CLKA and CLKB of line switch module 1a and those derivedfrom CLKA and CLKB of line switch module 1b of FIG. 2. Additionally,each of the circuits 16a, 16b of FIG. 8 generates a signaling framesignal SIGA, SIGB respectively. The signaling frame signals are alsosupplied to the PLL and control circuit 18 from the circuits 16a, and16b in both line switch modules.

Turning now to FIG. 12, the signaling frame signals SIGA and SIGB arecoupled to a 1 of 4 selector 1205. Each of the four 8 kHz inuts FS8A,FS8B are individually connected to a failure detector circuit 1201,10202, 1203, 1204. A 1 or 4 selector 1206 is to selct one of the clockoutputs from the failure detectors 1201, 1202, 1203, 1204 as the inputFS8 to the phase-locked loop 1207. The selector 1206 as well as selector1205 is controlled by the PLL controller 1250. The PLL 1207 is locked tothe selected input. If a failure should occur in the PCM line from whichthe signal FS8 is derived, service to the other PCM lines would beinterrupted if there was no capability of change from the PCM line towhich it is phase-locked. The failure detectors 1201, 1202, 1203, 1204detect failures in the PCM lines. FIG. 13 illustrates in detail thefailure detector 1201. Normally, a clock pulse at the 8 kHz rate of FS8Aarrives every 125 microsec. If, however, the time between two pulses isless than a predetermine interval, i.e., 100 microsec., the one shot1301 will prevent the fast pulse from being propagated. If the timeinterval between pulses is greater than 100 microseconds, (and less thana second predetermined interval as discussed below) the pulse will betransmitted through gates 1303 and 1037. If the time interval betweenpulses is greater than the second predetermined interval, i.e., 150microseconds, the one shot 1302 will time out, its Q output will go lowinhibiting gate 1303. Additionally, the PLL controller 1250 willmaintain a logic low on lead 1308 with the result that the clear inputto one shot 1302 is low. With the clear input low, the output Q ismaintained low. Thus, the one shot 1302 is "latched" in the low state.When Q is low, Q is high. Three bit binary counter 1305 is arranged suchthat when Q is low, binary 4 (ABC=001) is loaded and when Q goes high,the counter 1305 may begin counting. Counter 1305 has a clock inputcoupled to the 4.096 mHz clock CLK4. When the counter reaches binarycount 6 (ABC=011) a signal is provided at the output of gate 1307. Theoutput signal will persist through a binary count of 7 (ABC--111) andwill terminate at the next binary count (ABC=000) i.e., a binary countof 0. When a binary count 0 is reached, the output signal at gate 1307is terminated, a low is applied to the clear input of counter 1305causing counter 1305 to "latch-up".

Additionally, a signal is provided to the PLL controller 1250 on line1309 indicating that a failure has occurred. Thus, if a pulse is absentfor more than 150 microsec. a "phantom" pulse is provided and the PLLcontroller 1250 is informed of the failure. The PLL controller 1250 willselect a different one of the PCM lines to use as a reference for thePLC 1207. The phantom pulse is required to insure that the PLL 1207 doesnot miss a pulse. If no pulse were to occur, the PLL 1207 would functionas though it were an entire frame out of phase and would slow down the4.096 mHz clock CLK4 to correct for this. This would result in the rateconverter losing several frames of PCM before frame resynchronizationhad occurred. Since the phantom pulse is out of phase with a normalpulse stream, it does cause an aberration in the operation of PLL 1207.However the PLL 1207 has a narrow loop bandwidth such that is outputwill be within the limits of the rate converter tolerance. Theaberration in the PLL is less than 100 nanosec. of phase change.

A block diagram of the PLL 1207 is shown in FIG. 14.

The PLL 1207 comprises a clock input FS8 to which a voltage controlledoscillator (VCO) 1403 is slaved. This is accomplished by dividing boththe output of the VCO and the input clock to get a common frequency,i.e. 8 kHz. The signaling frame and frame clock generator 16a of FIG. 8divides the incoming 1.544 mHz PCM line clock CLKA to obtain an 8 kHzoutput FS8A which is in this example selected by selector 1206 of FIG.12 to provide signals on FS8. The divider 1404 of FIG. 14 divides the4.096 mHz output of VCO 1403 to also generate an 8 kHz signal. The phasedetector 1401 generates an error current which is integrated by the loopfilter 1402 to control VCO 1403. When CLK4 is phase-locked to FS8, thefrequency of VCO 1403 is exactly 512/193 times CLKA.

In order to minimize the phase variance in the PLL 1207, a phasedetector is provided that gives a phase error near zero under steadystate conditions. A simple phase detector that meets this criticalrequirement is shown in FIG. 15.

This phase detector requires TTL compatible inputs and unlike aquadrature or an EXCLUSIVE-OR type of phase detector, it is notduty-cycle dependent on the inputs since it is strictly rising-edgesensitive which prevents phase lock from occurring on an integermultiple (harmonic) of the desired VCO frequency. It similarly rejectssubharmonic phase lock.

The outputs (Q1 and Q2) of flip-flops 1501, 1502 have the following fourstates:

State 1=Both Q1 and Q2 high

State 2=Q1 is low, Q2 is high

State 3=Q1 is high, Q2 is low

State 4=Both Q1 and Q2 low

State 4 is an unstable condition since this causes a set pulse whichreturns the flip-flop pair to State 1. State 1 is the "off" state inthat both transistor switches (TR1 and TR2) are turned off in thatstate. FIG. 16 shows the state diagram of the phase detector.

The phase detector cycles through the state of diagram (FIG. 16)starting at State 1 progressing to either State 2 or 3, to State 4, andreturns to State 1. It makes this cycle once every 125 microsecondsstaying in State 1 for a large duration of the cycle. If F2 lags F1 inphase, the detector cycles through State 2; if F2 leads F1, it cyclesthrough State 3. The time it is in State 2 or 3 is equal to the phasedifference between F1 and F2. Thus, the phase detector has a+360 degreesof phase error operating range. If the error exceeds 360 degrees ineither direction or F2 is not equal to F1, the cycle time is notnecessarily 125 microseconds (out of lock condition). However, the phasedetector will always cycle through State 2 when F2 is less than F1 andthrough State 3 when F2 is greater than F1. Therefore, the phasedetector will, in all cases, drive the control voltage toward a lockedcondition.

When in State 2 or 3, TR1 or TR2 is "on" causing current to be pumpedinto or out of the loop filter capacitor (C). When fed into aninfinitely high impedance, the voltage on the capacitor represents thesummation (or integral) of all past phase errors. This is extremelyimportant, since different VCO's require different control voltages tooscillate at 4.096 mHz. The phase detector will operate at nearly zerophase error under steady state conditions independent of the DC voltageon the control voltage line. TR1 and TR2 need only to be turned on tocorrect for changes in PLL operating conditions and to replace the smallamount of charge lost in the capacitor due to circuitry leakage duringeach 125 microsecond period. The phase error is independent of the vastmajority of the PLL parameters including loop gain, supply voltage, loopfilter values, and all VCO characteristics. In comparison, when usingphase detectors in which the control line voltage is directlyproportional to the phase error, the phase error is much more difficultto control. With such phase detectors any required change in controlvoltage (due to part variance or temperature change) causes aproportional phase error change.

Phase error between the VCO and its reference is dependent primarily onthe propagation delay through the dividers (512 and 193) and the phasedetector. The dividers were both designed with this consideration andhave only one flip-flop delay from input to output. The ÷193 counter isthat of FIG. 11 and the -512 counter is shown in FIG. 17. The phasedetector uses two flip-flops that are inherently closely matched sincethey are in the same integrated circuit package.

The PLL has a measured nominal phase difference between F1 and F2 at thephase detector of 10 nanoseconds (less than 0.03 degrees). The totalphase diffrence between CLK4 and CLKA or CLKB will be larger since thereare additional circuit delay tolerances that affect the total phasedifference. These include delay tolerances in the dividers, failuredetector circuits, selectors and buffering circuits. The worst casephase difference between CLK4 and CLKA or CLKB is under 200 nanosecondsusing LSI-TTL technology.

The phase/frequency discrimination attributes of the phase detector givethe PLL a capture and lock range limited only by the frequency range ofthe VCO. This allows great flexibility in the design of the remainingportions of the PLL.

The design of the loop filter 1402 of FIG. 14 involves severalrequirements. First, the loop must remain stable under all operatingconditions. Secondly, the loop bandwidth must be selected such that theaberration in the 8 kHz reference caused by a PCM line failure, does notcause the VCO to shift in frequency excessively. A damping factorgreater than 0.7 is desirable for stability. A damping factor under 0.7is underdamped with zero being the limit before oscillatons results. Forstability analysis, the PLL can be modeled as shown in FIG. 18.

Using Laplace Transforms, it is possible to analyze the stabilty of thePLL. Using servo theory, ##EQU1## where βo(s)=Output phase

βi(s)=input phase

G(s)=Forward gain product=K_(O) ×K_(F) ×K_(V) /s

H(s)=Feedback gain product=l/N

Three of the gain blocks (Ko, Kv, and N) are fixed. The transferfunction of the loop filter is selectable. The loop filter of FIG. 19 ischosen for its great flexibility in determining both loop naturalfrequency (μn) and damping factor (ζ). ##EQU2## where τ₂ =R₂ C

τ₁ =(R₁ +R₂)C

Referencing FIG. 18, forward gain and reverse gain products are enteredinto Equation 1 and reduced to Equation 4. ##EQU3##

Table 1 lists the values of the PLL parameters used to determine theloop damping factor (ζ) and natural frequency (μ_(n)).

                  TABLE 1                                                         ______________________________________                                        PLL GAIN PARAMETERS                                                           ______________________________________                                        K.sub.V = 550 kHz/volt = 3.46 × 10.sup.6 rad/volt-sec.                   K.sub.0 =                                                                               ##STR1##                                                             N =     512                                                                    K.sub.F =                                                                             ##STR2##                                                           ______________________________________                                    

These are used in Equation 4 to obtain the following:

w_(n) =158 rad/sec. (2.51 Hz)

ζ=1.09

This indicates a PLL with a narrow loop bandwidth and is very stablesince it is overdamped.

Two loop-pass sections were added to the basic loop filter shown in FIG.19 in order to reduce the aberration caused by a PCM line failure. Inaddition, isolation is provided between the VCO and active loop filtercircuitry at the VCO operating frequency of 4.096 mHz. Turning to FIG.19A which illustrates the loop filter in simple form, the cornerfrequencies of the two low-pass filters R3, Cl and R4, C2 is more thanan order of magnitude higher in frequency than the natural frequency ofthe PLL. Thus, the additional phase shift contributed by these filtersis negligible and lowers the damping factor only slightly. A J-FET inputop amp- 1901 is used to provide the low leakage required since the VCOhas an input impedance under 100K ohms. FIG. 20 illustrates in detailthe complete phase-locked loop circuit combining the circuits of FIGS.15, 17 and 19A. An unlock indication circuit 2003 provides a signalindicating if the phase-locked loop fails to maintain a locking.

The choice of the VCO is often the most important part of the design ofa phase-locked loop. However, in this application, the PLLcharacteristics that the VCO determine are not critical. This includesphase noise, voltage to frequency conversion linearity, temperaturesensitivity, frequency range and spectral purity. Three different VCO'swere investigated. All were IC designs available from multiple sourcesand all met the requirements necessary for this applicaton. The MC4024and 74LS124 are TTL multivibrator designs using an RC network todetermine frequency. These parts have very similar performance withapproximately 4 nanoseconds of phase jitter at the phase detector. Thiscorrelates with the MC4024 specification of 120 Hz RMS (typical)frequency noise deviation at 4.096 mHz. The MC1648 VCO is an ECL ICoscillator design requiring an external varactor and coil. Because ofthe higher Q elements, it has superior phase noise and spectral purity.At 4.096 mHz, the RMS frequency deviation is specified at less than 20Hz. This corresponds to a phase jitter at the phase detector of lessthan 1 nanosecond. With the test equipment available, no phase jittercould be detected.

Turning back to FIG. 12, the output of the PLL 1207 is coupled to a oneshot 1209 which acts as a failure detector. If the PLL 1207 fails to bereset within 350 nonosec., it will set flip-flop 1210 and will provide afailure indication to the PLL controller 1250. Flip-flop 1210 willautomatically operate the selector 1214 to switch to the phase-lockedloop in the other line switch module, e.g., lb of FIG. 2. The PLLcontroller 1250 must however switch the PCM line clock, e.g., CLKA, tothe PLL 1207 in the other module without missing clock pulses. The 500nanosec. delay 1208 permits switching to the other PLL without missingclock pulses since the delay 1208 provided clock pulses for 500 nonosec.after the first PLL 1207 has failed. The selectors 1212 and 1213 arealso controlled by flip-flop 1210 to switch the source for SIG and FRAMEto the other line switch module.

The PLL controller 1250 includes a microcomputer 1251 and an interfacecircuit 1252. The microcomputer 1251 in the illustrative embodiment isan INTEL 8049 microcomputer. The interface comprises a protocolinterface circuit (PIC) which will be described in greater detail inconjunction with the line group controller. The INTEL 8049 microcomputeris described in INTEL COMPONENT CATALOG 1979, INTEL CORPORATION, 1979,pp. 8-27 to 8-30.

The system controller 5 selects which PCM line clock is to be initiallyselected as the master clock to which the line switch clock circuits areto be synchronized. The system controller 5 informs the line switchcontrollers 7 in the line switch of the initial selection. The lineswitch controllers 7 then direct the respective microcomputers 1251 tothe initial selection. If one of the line module microcomputers 1215detects a failure in the clock circuits, it arbitrarily switches toanother line clock to be used as a master. The failure detectingmicrocomputer will send information back to the system controller 5 vialine switch controller 7 identifying the newly selected PCM line clock.The system controller 5 will then transmit command information to theother modulemicrocomputer 1251 via the respective line switch control 7directing the other modules microcomputer to select the same PCM lineclock as master. Thus, all modules within the line switch utilize thesame PCM line clock to derive the internal line switch clocks.

5.0 LINE SWITCH CONTROLLER (FIGS. 9B, 9E to 9N, 10B 10C)

The line switch controller 7 communicates with the system controller 5of FIG. 1 via the PCM lines PCMA, PCMB. The line switch controller 7transmits and receives information over the common channeling signalingbits of the PCM lines PCMA, PCMB via the RX and TX rate converters. Morespecifically, turning to FIG. 9B, each time a common channel signalingbit appears on the PCM line, PCMA or PCMB, the associated clock lead FSAor FSB will provide a pulse. The clock pulse on the lead, e.g. FSA willcause the common channel signaling bit to be loaded into a bufferflip-flop 950A and 950B. The output of buffer flip-flop 950 or 950B ispresent to the line switch controller via lead CCRXA or CCRXB.Information from the line switch controller 7 is inserted into thecommon channel signaling bit portions on lines PCMA and PCMB undercontrol of the clock leads FS8A, FS8B. Turning to FIG. 10B, commonchannel signaling information from the line switch controller 7 for PCMline PCMA is provided on lead CCTXA and for PCM line PCMB on lead CCTXB.Tri-state buffer gate 1086 is enabled by CLKA during the common channelsignaling bit time and gates the data bit from line CCTXA into PCM linePCMA. Likewise, data from laad CCTXB is gated onto PCM line PCMB.Turning now to FIG. 9E, the line switch controller is shown ascomprising a Protocol interface circuit PIC and a microcomputer. Themicrocomputer utilized in the illustrative embodiment is the MCS-85^(TM)system available from INTEL CORPORATION which is described in INTELCOMPONENT DATA CATALOG 1979, Intel Corporation, 1979, 9-79 to 9-139.More specifically, the microcomputer configuration may be that shown inFIG. 1 at page 9-78 of the aforementioned Intel reference. The 8085microprocessor, the 8156 RAM with I/O ports and timer, the 8355 ROM andthe 8185 RAM shown are all described in detail in the aforementionedINTEL reference.

The protocol interface circuit PIC serve as a control data interfacebetween the parallel data bus of the microcomputer and the high speed,bit serial PCM bus PCM4. The line switch controller 7 operates as themaster in a master slave relationship with all other circuits connectedto the PCM buses PCM4, PCMLCA, PCMLCB. More specifically, only the lineswitch controller 7 may transmit control word commands over the PCMbuses. A circuit will transmit a control word response on the PCM busesonly after receiving a command addressing that circuit. The conventionestablished for the line switch control communication specifies thatcommands and responses must occur in pairs, only a line switchcontroller 7 can initiate a command, and the response to a commandalways occurs a fixed number of frames after the command.

FIG. 9F is a flow chart of the general software architecture of the lineswitch controller 7. Commands from the system control 5 of FIG. 1 aretransmitted over the PCM lines PCMA, PCMB arriving at a rate of one byte(8 bits) every 2 msec. The line switch controller includes a 2 Msec.interrup timer and when a 2 msec. interrupt occurs, an interrupt handlerprogram calls a common channel data handler to process the received databyte. If the received data byte completes a command (each command mayhave 5 to 10 bytes), the previously received bytes for the command willbe retrieved from a queue and the command will be executed. If thereceived byte does not complete a command, it will be stored in thequeue. Typical of the tasks which the line switch controller 7 will becommanded to execute are: calling and called party channel assignments,initiating ringing, disconnecting ringing, call disconnect maintenance,test, alarm and administrative functions. FIGS. 9G to 9N illustrate ingreater detail the flo charts of various programs identified in FIG. 9F.

FIG. 9G illustrates the interrupt handler and common channel handleprograms.

The following notes are appropriate to FIG. 9G.

EOT indicates end of test.

REV BUFF indicates receive buffer and is the first location of temporarystorage for common control commands.

RES PTR (response pointer) is set to the character in a command channelresponse or command.

RES Flag is set by the command channel routine when a response is to besent.

RCU PTR is a receive pointer.

COMM FLAG indicates when a complete command has been received.

NON-SGAL CTR indicates when 5 consecutive non-signal codes have beenreceived, this initializing resynchronization.

INT SERVFLAG indicates that a 2 millisec. interrupt has occurred.

RESP DLY CTR delays characters to 300 baud.

FIGS. 9H to 9L are flow charts for various task routines, FIG. 9M is aflow chart of the ringing administration program; FIG. 9N is a flowchart of the line scan routine.

6.0 BUFFER AND DISTRIBUTOR 9 (FIGS. 21, 22, 23)

FIGS. 21 and 22 illustrate the buffer and distributor circuits 9 of FIG.2 in greater detail. For purpose of clarity, only one buffer anddistributor 9 for each line switch module 1a 1b is shown in FIGS. 21 and22. If in accordance with the aforementioned security block concept eachline switch module 1a, 1b includes more than one buffer and distributor9, then one buffer and distributor 9 of each line switch module 1a, 1bwill include both the circuitry of FIGS. 21 and 22 and the other bufferand distributor 9 of each line switch module 1a, 1b will include onlythe circuitry of FIG. 21. The circuitry of the buffer and distributor 9for each of the line switch modules 1a, 1b shown in FIGS. 21 and 22 areidentical and corresponding terminals of the buffer distributor circuitshave identical designations. FIG. 21 illustrates the circuitryassociated with the PCM buses PCM, PCMLCA, PCMLCB and FIG. 22illustrates the clock distribution circuit.

6.1 BUFFER (FIGS. 21 and 23)

Line switch module 1a of FIG. 21 has terminals 707 and 708 connected toPCM bus PCM4, terminals 701 and 702 connected to PCM bus PCMLCA andterminals 704 and 705 connected to bus PCMLCB. Interposed betweenterminals 707 and terminal 701 are gate 740, shift register 741 andbuffer gate 742. Interposed between terminals 702 and 708 are gates 743and 744. A path is provided between the output of gate 744 to the leadbetween gate 740 and shift register 741 via gate 745 and delay circuit746. Gates 740 and 746 are controlled by gate 747 through delay circuit746a. Gate 747 has one input connected to terminal 703 and another inputconnected to terminal 715. Gates 740 and 745 are arranged such that gate740 is normally closed and gate 745 is normally open, but when anappropriate signal state is present at terminals 703 or 715 gate 740 isopen and gate 745 is closed. Together gates 740 and 745 operate as acrosspoint switch which is utilized for intra-line switch calls asdescribed below.

Terminals 704 and 705 are respectively coupled to terminals 714 and 713via gates 748 and 749. Terminal 706 is connected directly to terminal712.

The two line switch modules 1a, 1b are interconnected via intra-lineswitch highways 1H1 and 1H2 and leads kILSB and ILSA. It should be notedthat the gates 743 and 748 include an additional input CTRL which may beindividually controlled in the event of a fault to inhibit PCMtransmission.

When the system controller 5 (shown in FIG. 1) determines that asubscriber connected to a line circuit has requested a connection toanother subscriber connected to a second line circuit whithin the sameline switch, the system controller 5 forwards commands to line switchcontroller 7 shown in FIG. 2 indicating that an intra-line switchconnection is required. The line switch controller 7, in turn, providescontrol signals to the line circuits of the originating and terminatingparties thereby loading status bits in the respective line circuits. Thestatus bits in turn provide gating of control signals AILS, BILS to thecrosspoint switches 740, 745 of FIG. 21 associated with the originatingand terminating line circuits. The respective AILS and BILS signals areactive only during the time period when the transmit channels occur,which the line switch controller 7 has assigned to the originating andterminating line circuits.

Turning now to FIG. 23, the operation of switches 740, 745 is described.FIG. 23 shows the two line switch modules 1a, 1b each connected to thecorresponding interface logic 6. In line module 1a, only two linecircuits 9A and 9B are shown and in line module 6 only two line circuits9C and 9D are shown. It should be understood that only two line circuitsare shown in each line switch module 1a, 1b for purposes of clarity.

Two intra-line switch call operations will now be described, withreference to FIG. 23. First, assume that the subscriber A connected toline circuit 9A has requested a connection to a subscriber B connectedto line circuit 9B, i.e., to a subscriber connected to the same linemodule. The system controller 5 of FIG. 1 provides control signals toline switch controller 7 of line switch module 1a, that an intra-lineswitch call between line circuits 9A and 9B has been requested. Linecircuit controller 7 transmits commands to the line circuit groupscontaining line circuits 9A and 9B, indicating that an intra-line switchcall connection is to be established, and that bus PCMLCA is to be used.The transmit and receive channels for the two subscribers A and B arealso assigned. When the transmit channel for subscriber A occurs, linecircuit 9A provides an active signal on lead AILS which closes switch745 and opens switch 740. The transmit lead of PCM bus PCMLCA connectedto terminal 702 is thus connected to the receive lead of PCM bus PCMLCAconnected to terminal 701. Thus, a PCM sample from subscriber Atransmitted by line circuit 9A on the transmit portion of bus PCMLCA is"looped back" to the receive portion of bus PCMLCA. The receive channelassigned to subscriber B line circuit 9B is assigned such that itcorresponds to the transmit channel of subscriber A of line circuit 9A.Therefore line circuit 9B receives the PCM sample transmitted by theline circuit 9A. Likewise, when the transmit channel for line circuit 9Boccurs, line circuit 9B will apply an active signal to line AILS. Switch740 will again open and switch 745 will again close for the duration ofthe transmit channel. Thus, the PCM sample transmitted by line circuit9B on the transmit portion of PCM bus PCMLCA is "looped back" to receiveportion of PCM bus PCMLCA. The receive channel assigned to line circuit9A corresponds to the transmit channel assigned to line circuit 9B.Therefore, line circuit 9A will receive the PCM sample transmitted byline circuit 9B.

Second, assume that the subscriber A connected to line circuit 9A hasrequested a connection to a subscriber D connected to line circuit 9D,i.e., to a subscriber connected to a different line switch module withina line switch. The system control of FIG. 1 provides control signals toline switch controller 7 of FIG. 2 of the line switch module 1a that anintra-line switch call between line circuit 9A of line module 1a andline circuit 9D of line switch module has been requested and bus PCMLCAis to be used.

Line switch controller 7 of line switch module 1a assigns transmit andreceive channels to line circuits 9A and 9D of FIG. 23. Again theassigned transmit channel of line circuit 9A will correspond to thereceive channel of line circuit 9D and the assigned transmit channel ofline circuit 9D will correspond to the receive channel of line circuit9A. The crosspoint switches 740 and 745 are then operated in the samemanner as described above, with the exception that line circuit 9Dprovides active signals to control the switches 740 and 745 instead ofline circuit 9B.

6.2 DISTRIBUTOR (FIGS. 22 and 24)

The clock distribution circuitry of the buffer and distributor circuits9 for two line switch modules is shown in FIG. 22. Each buffer anddistributor 9 has input terminals 709, 710, 711 connected to lines FS,CLK4, and SIG from a corresponding interface logic 6. A clock generatorcircuit 770 generates a CODEC clock signal CLKT and a frame clock signalFSD. The outputs FSD and CLKT are coupled to terminals 712 and 713respectively via controlled switches 760 and 761 and are directlycoupled to terminals 721 and 720. Input terminals 718 and 719 arecoupled to terminals 712 and 713 respectively via controlled switches762 and 763. Each of the switches 760, 761, 762, and 763 has a controlinput commonly connected to lead GEN CTRL and operates such that when asignal having a first state is present on GEN CTRL, switches 760 and 761are open and switches 762 and 763 are closed. When a signal having asecond state is present on GEN CTRL switches 760 and 761 are closed andswitches 762 and 763 are open. By means of the switches 760, 761, 762,763, the FSD and CLKT signals for a line switch module may be suppliedeither by the clock generator 770 within the line switch module or bythe clock generator in another line switch module. The GEN CTRL lead ofa line switch module is connected to the line switch controller in thesame line switch module.

Timing signal waveforms for the signals FS, CLK4, FSD and CLKT are shownin FIG. 24. A 9 stage counter within the clock generator 770repetitively counts 512 4.096 mHz pulses before recycling.

The FS signal received from the interface logic 6 of FIG. 2 is a framesynchronization pulse which occurs every 125 microseconds. CLK4 is a4.096 mHz clock signal from interface logic 6. SIG is likewise providedby interface logic 6 of FIG. 8 and indicates the occurrence of signalingframe, i.e., SIG is normally low and goes high the frame before asignaling frame. CLKT is CODEC clock which generally comprises bursts ofeight pulses of a 4.096 mHz clock. The relationship of CLKT to the CODECchannels is shown. When a frame bit FSD occurs, the next 32 bitsreceived over the PCM bus are control bits. More specifically, in asystem with 10-bit channels, the next three channels are reserved forcontrol data and channel three is extended by two bits. In a system with8-bit channels, four channels are reserved for control.

FSD is a framing signal which will vary as shown depending on whether itoccurs during a signaling frame or not.

Turning back to FIG. 22, the CLK4 lead coupled to terminal 710 isconnected to output terminal 774 via switch gate 772 and is connecteddirectly to output terminal 773. Input terminal 774 is coupled to outputterminal 775 via switch gate 776. The terminal 774 is connected toterminal 773 of the other line module. The switch gates 772 and 776operate in the same manner and under control of the same GEN CTRL signalas the aforementioned switch gates 760, 761, 762, 763 so that eitherCLK4 from line module 1a or 1b may be utilized as the source for 4.096mHz clock signals on lead CLK4LS.

7.0 LINE CARD OR GROUP (FIG. 25)

One of the line groups 11 of FIG. 2 is shown in detail in FIG. 25. Eachline group includes a common circuit portion 31 and eight line circuits32 in the illustrated embodiment. In other embodiments, the commoncircuit portion 31 may be on a separate circuit card and a line groupmay have more or less than eight line circuits. The common circuitportion includes a control interface 33 which is coupled to PCM busPCMLCA via buffer 901 and switch gate 902, and to PCM bus PCMLCB viabuffer 903 and switch gate 904. The control interface is also coupled tothe FSD and CLKT leads via buffers 905 and 906, respectively. Theinterface 33 extracts and inserts control data bits which aretransmitted and received over PCM buses PCMLCA and PCMLCB. Control datareceived over the buses are examined to determine if the addressreceived matches the address of the line group. If the received addressmatches, an interrupt signal is sent via line INT to a line cardprocessor 34 which may be a conventional microprocessor of a type wellknown in the art. It should be noted that the line group address, whichthe control interface 33 matches against is, in fact, supplied by theline group processor 34 and is stored by the control interface 33. Theline group address is supplied to the processor 34 by leads 35 toprovide a unique address to the line card.

Other control data are communicated between the processor 34 and the PCMbuses PCMLCA, PCMLCB via the control interface 33, which acts as a databuffer. It should be noted that the control interface 33 is providedbecause the microprocessor 34 cannot operate at a high enough rate tohandle 4.096 mbps serial rate of the PCM buses. In other applications,which do not utilize a high speed serial bus for control information,e.g., where control information is received in parallel, or at a slowerrate, the control interface 33 may not be necessary. In other words, themicroprocessor 34 may be directly coupled to the bus carrying thecontrol information.

After the processor 34 determines that an operation is required withinits associated line group, it will select the appropriate line circuit,as determined by portions of the control data received over PCM busPCMLCA or PCMLCB.

The processor 34 is connected to the line circuits 32 via bus 36. Bus 36includes ten separate leads, specifically, a separate clock leadCLK/SWHKn (n=1 to the number of circuits in a group) for each of theline circuits, a bidirectional data lead DATA, and an address latchenable lead COMMAND. Control and data information is transferred betweenthe line group processor 34 and the per line control interfaces 44 overthe common lead DATA. The information transfer is controlled by theCOMMAND lead and the CLK/SWHKn leads.

Additional leads connected to the line group are CLK4LS, which is the4.096 mHz clock signal, leads AILS, BILS, and SUPVY. The lead CLK4LS isdistributed to the line circuits via a buffer gate 907. The per linecontrol interface 44 of each of the eight line circuits 32 generates thesignals AILS, BILS and SUPVY. The AILS outputs of the line circuits are"wire-ored" to the input of buffer output gate 908. Likewise the BILSoutputs are "wire-ored" to gate 909 and the SUPVY outputs are"wire-ored" to gate 910.

The 4.096 mHz clock ClK4LS is applied to a divide by two circuit 941which provides a 2.048 mHz clock signal CLK2 for use by the linecircuits.

7.1 LINE CIRCUIT (FIGS. 25, 26, 27)

Each line circuit 32 includes conventional line circuit transmissioncomponents: test and ring relays 38 and 39, a subscriber line interfacecircuit 40, a filter circuit 42 and a codec 43. The CODEC 43 is of aknown type which is operable in a so called "microprocessor controlledmode" of operation, wherein the channel assigned by a line switch orsystem controller to the associated line circuit is stored by the CODECand is used to determine when the CODEC will become active. The CODECmay be defined as "active" when the CODEC is transmitting or receivingover PCM buses. One such CODEC is commercially available from the INTELCorporation and is identified as type number 2910A.

Each line circuit 32 further includes a per line control interface 44which interfaces between the common circuit 31 and the line circuittransmission components. Control of portions of the per line controlinterface 44 is achieved by the processor 34 transmitting control wordsover bus 36. Each of these control words is 12 bits in length andarranged in format as shown in FIG. 26. Bit 0 determines whether thecontrol operation is a read or write; bits 1-3 are address bits for theper line control interface 44; bit 4 is unused; bit 5 is set accordingto whether the line card is in a system using 8 bit or 10 bit PCM wordson its buses; bits 6, 7 and 8 are used to control external componentssuch as power control, device test relay, and ring relay; bit 9 enablesthe line circuit; bit 10 is used to signify intraline switch calls; andbit 11 selects which of the two PCM buses PCMLCA or PCMLCB will beutilized by the line circuit.

One of the per line interfaces 44 of FIGS. 25 is shown in greater detailin FIG. 27. The CLK/SWHKn lead is a dual function lead. Morespecifically, one function of this lead is that of a normal clock leadto clock data into or out of the line interface circuits 44. The otherfunction of the CLK/SWHKn lead is to return switch hook status to theline group microprocessor 34 of FIG. 25. The selection as to which ofthe above two functions is provided is determined by the state of theCOMMAND lead and register 48. When the COMMAND lead is high, and whenbit 0 of the control word indicates a read, then gate 46 enablestri-state buffer 47 thereby coupling the lead SWHKn, which is connectedto the SLIC 40 of FIG. 25 and indicates current switch hook status, tothe line card processor 34 of FIG. 25 via CLK/SWHKn lead. When theCOMMAND head is low, gate 45 is enabled and control bits from the DATAlead of bus 36 are shifted into shift register 48. After the fourcontrol bits 0-3 are loaded in register 48, the COMMAND lead goes highdisabling gate 45 thereby holding the control bits in register 48. Gates49 and 50 form a decoder circuit to gate clock pulses from the CLK/SWHKnlead to shift register 53. Similarly, gates 51 and 52 determine whetherdata will be stored in or read from register 53. If data is to be storedin register 53, the next eight bits, i.e., bits 4-11 of the control wordare shifted into the register 53 from the Data lead. The A/B select leadcontrol gates 54, 55, 56, 58 to connect either bus PCMLCA, or PCMLCB tothe coded PCM bus 64A, 64B. Flip-flops 65 provides buffer timing for thetransmit PCM signal from bus 64A.

The TSX lead provides timing from the CODEC 43 of FIG. 25. Gate 66controls gating of the TSX signal into buffer flip-flop 67. The Q outputof flip-flop 67 is connected to gates 60 and 61 which are controlled bythe A/B select bit to steer the TSX signal to leads TSXA or TSXB. The Qoutput of flip-flop 67 is also coupled to gates 62 and 63 which arecontrolled by the A/B select bit and the ILS bit. The outputs of gates62 and 63 are coupled respectively to leads AILS and BILS.

In some applications it may be desirable to return switch hookinformation at a rate fast enough to detect dial pulsing withoututilizing the A/B signaling bits of the PCM bus. To accomplish this,switch hook information from lead SWHK from the line circuit 32 is gatedonto the supervisory lead SUPVY during one half of the channel timeassigned to that circuit. The channel during which switch hookinformation is gated is determined by a signal on lead TSX from theCODEC 43 of FIG. 25.

A counter 74 divides each channel into two parts. Flip-flops 82, 83provide synchronization for counter 74. The A/B select bit controls gate75 to determine in which of the two parts of a channel the switch hookinformation is to be gated. Gates 76 and 77 gate the switch hookinformation from flip-flop 67 to supervisory lead SUPVY. The linecircuit enable bit controls gating of the lead TSX at gate 66 formaintenance purposes, i.e., if it is determined that CODEC 43 of FIG. 25is defective, the line circuit enable bit is utilized to prevent theCODEC 43 from interfering with the operation of the remainder of thesystem. The ring relay, test relay and power control flip-flops 80, 79,78, respectively control the application of ringing signals from bus 73of FIG. 14 control connection to a test bus and to apply power to theentire transmission circuit elements of FIG. 14. Register 81 delays theframe clock FSD from the buffer-distributor 9 of FIG. 22 by one channeltime to provide a receive frame clock which is coupled to a CODEC 43 ofFIG. 25 via lead FSDRX. The Euro Select bit, i.e., bit 5 of FIG. 26,determines whether the channel delay corresponds to 8 or 10 bits.

7.2 GAIN/BALANCE (FIGS. 25, 28, 29, 30, 31)

Turning back to FIG. 25, the gain/balance control circuit 41 providessoftware selection of transmit gain, receive gain and balance for theline circuit. Control of the gain/balance circuit 41 is achieved by theline card processor 34 transmitting control words over the serial databus 36 in a manner similar to that described with respect to the perline controller interface circuit 44 hereinabove. The control words areillustrated in FIGS. 28 and 29. As shown in FIG. 28, a control word forthe gain/balance operations comprises 28 bits arranged as follows: aread/write bit, three address bits, a clock control bit, three unusedbits, four bits for line balance network selection and eight bits eachfor transmit and receive gain selection. As shown in FIG. 29, a controlword for CODEC control comprises 12 bits which includes a read/writebit, three address bits, two mode bits and six channel assignment bits.

The gain/balance circuit 41 is shown in detail in FIG. 30. The firstfour bits of a control word are loaded into and stored by register 84 inthe same manner as described for register 48 of FIG. 27. Addressdecoders 85, 86, 109 and 87 decode the contents of the register 84.Decoder 85 determines if the control word is to provide gain/balancesettings to be stored in register 88. Decoder 86 permits loading ofassigned channel information into a register 89. Decoder 87 controls theloading of channel assignment information from register 89 into theCODEC using data clock lead CLKD.

If the first four bits of the control word shown in FIG. 28 stored inregister 84 as indicated above activate decoder 85, the next 24 bits arethen stored in register 88. The clock control bit stored in bit Q23 ofregister 89 controls the state of flip-flop 92 which in turn isconnected to EXCLUSIVE-OR gate 93 to determined whether the clock signalon lead CLKD is inverted or not. The four line balance network selectionbits of shift register 88 control analog solid state switches 94 toselectively connect capacitors 95 and 96, network 97 and 98, to the SLICcircuit via the balance selection lead BAL. It should be noted that thecapacitors 95 and 96 and resistor 98 may be replaced with other networkcomponents or with entire networks.

Gain control amplifiers 103 and 105 are connected such that they areinterposed in the transmission path between the SLIC 40 and Filter 42 asshown in FIG. 25. More specifically, receive amplifier 103 has one inputconnected to lead RXF connected to the receive filter and an output leadRXS connected to the receive path of the SLIC. Amplifier 105 has oneinput connected to SLIC transmit lead TXS and an output connected totransmit filter lead TXF.

The eight receive gain control bits stored in register 88 control analogsolid state switches 101 to selectively connect resistors 102 to oneinput of amplifier 103. Likewise the eight transmit gain control bitsstored in register 88 control analog switches 104 to selectively connectresistors 106 to one input of amplifier 105.

It is desirable to maintain closely controlled gain steps, e.g., lessthan 0.1 db, over a wide range of environmental conditions. Typically,gain circuits comprise a resistive ladder similar to that formed byresistors 106, each leg of the ladder having an analog switch connectedin series with a resistor. The ladder in turn forms a resistive dividerwith another resistor 107. The analog switches have a finite impedancewhich will vary from unit to unit and with environmental changes.Typically, analog solid state switches have impedances of nominally 50to 150 ohms in the on-state. The variation of the on-state impedancewould thus contribute a significant change to the ratio of the dividerspreviously used.

This problem is significantly reduced in the gain control circuits shownin FIG. 30. Specifically, an amplifier having an extremely high inputimpedance such as a FET differential input amplifier, e.g. 103 or 105,is connected in the divider network. The resistance values may then bemade relatively high, for example, in the order of tens of thousands ofohms, and accordingly, the impedance variations in analog switches,which are on the order of a hundred ohms, will have a negligible effecton the gain through the circuit.

The gates 91 are controlled by the read/write bit of the control word todetermine whether a portion of a control word is to be stored inregister 88, or whether the contents of register 88 are to be read viathe data bus 37.

A CODEC control word has two distinct addresses. One address(hereinafter first address) will result in either the register 89 beingloaded or read by the line card processor 34, and the other address(hereinafter second address) will cause the contents of register 89 tobe loaded into a CODEC 43. If the control word contains the firstaddress and the read/write bit indicates a write operation, the mode andchannel assignment portion of the control work is loaded into register89 via decoder 86. If the control word contains the first address andthe read/write bit indicated a read operation, the contents of register89 are supplied to data bus 37 via gate 108, which is controlled byportion of the decoder 87. When the register 89 is read, the gates 109will feed each bit, as read, back into the register such that after aread of the register 89, the contents of register 89 are unchanged.

If the control word contains the second address, the contents ofregister 89 are supplied to the CODEC over data bus 37 via gate 108.Gate 108 is controlled by portions of the decoder 87. Other portions ofdecoder 87, when enabled, supply clock pulses to a CODEC via gate 93 andline 90 to clock the data from bus 37 into the CODEC. The gates 109 willagain operate to loop the data bits loaded into the CODEC from register89 back into register 89.

As noted hereinabove the control words transmitted between the linegroups and the line switch controller 7 of FIG. 2 over the buses PCMLCAand PCMLCB as shown in FIGS. 2 and 6 comprise 32 bits. FIG. 31illustrates the format of these control words. Each control wordtransmitted from the line switch controller 7 of FIG. 2 comprises foureight bit bytes, indicated as Bytes 1-4. Byte 1 may include one paritybit which is calculated over the entire word, and includes seven addressbits. Byte 2 includes five function bits which control the function thatline card processor 34 is to perform, and three bits to designate whichone of the eight line circuits on a line card is to be affected. Bytes 3and 4 contain control data as required for the function specified by thefunction bits. After a control word is transmitted to the line card bythe line switch controller 7, the processor 34 will respond by returningthe same control word back to the line switch controller 7 if thecontrol word indicates a write operation. If the control word indicatesa read operation, the line card processor 34 will return bytes 1 and 2as originally transmitted from the line switch controller 7 and willinclude respose data in Bytes 3 and 4.

7.3 PROTOCOL INTERFACE CIRCUIT/CONTROL INTERFACE 33 (FIGS. 25, 32)

The control interface 33 of FIG. 25 comprises the protocol interfacecircuit PCI and as shown in detail block diagram form in FIG. 32 isconnected to buses PCMLCA and PCMLCB. For purposes of clarity, singlelines are used to represent multi-line buses and the number of lines ina bus is indicated at various points along the bus.

Initially, the line group processor 34 of FIG. 25 reads the line cardaddress from leads 35 as shown in FIG. 25 and loads this address intothe control interface latch 123 of FIG. 32 in the following manner. Theprocessor 34 transmits the address to the latch 123 over the DATA/ADDlines to the control interface 33. This internal register address isbuffered by buffer circuit 120 and is presented to the input of register121. The line card processor 34 concurrently transmits a signal over theALE lead which causes the internal register address to be stored inregister 121. The internal register address decoder 122 decodes theaddress to apply an enable signal WR FIXED ADDR on lead 124. The linecard processor 34 then transmits a data word which contains the linecard wired address. The data word is applied to the input of register123 via bus 125 and is gated therein via lead 124 by address decoder 122upon receipt of a strobe signal from the processor or write enable leadWE. The register 123 then contains the wired address of the line card.

A timing generator circuit 127 receives the FSD and CLK4LS clock signalsfrom the buffer distributor 9 of FIG. 21 and generates various timingsignals for controlling the operation of the control interface 33. TheFSD signal is used to control the timing of a clock signal on lead S/Rclock. The clock signal on S/R clock controls the storing ortransmitting of control words between bus PCMLCA and register 128 andbetween bus PCMLCB and register 129. Initially, a control word isreceived on one of the two buses, e.g., PCMLCA. As the control wordappears on bus PCMLCA in serial form it is gated via selector circuit130 to shift register 128 using S/R clock. Registers 128 and 129 are 32bit registers. After the proper number of clock pulses have beenprovided by S/R clock, a strobe signal is applied to the ADDR MATCH ENAlead thereby enabling a comparator 131 which compares the addressportion of the control word in register 128 with the line group addressstored in latch 123 and generates a signal on lead 132 if the addressesare the same. Similarly control word received on bus PCMLCB are storedin register 129 via selector 145 and the address portion thereof iscompared to the line circuit address by comparator 133 which generates asignal on lead 134 if those addresses are the same. Gate 155 logically"or's" lead 132 and 134 and will provide a signal to the line groupmicroprocessor 34 of FIG. 25 on its interrupt lead INT if an addressmatch occurs for either bus PCMLCA or PCMLCB.

After the line group processor 34 is interrupted it will then obtain thecontrol word by first transmitting an address over the DATA/ADD bus. Theaddress is stored in latch 121 and decoded by decoder 122 which, in turnactivates the STATUS RD line. The STATUS RD line enables gates 135 and136 which provide indications on the DATA/ADD bus as to which of the twocomparators 131 and 133 detected the line card address.

The line group processor 34 will then, via a series of commands, obtain24 bits of the control word from the appropriate one of the registers128 or 129. If the control is obtained from register 128, thenmultiplexer 137 and buffer 138 are utilized. Similarly if the controlword is to be obtained from register 129 then multiplexer 139 and buffer140 are utilized.

Response data from the line group processor 34 may be returned to theline switch controller 7 via buses PCMLCA or PCMLCB.

Response data from the line group processor 34 is stored in apreselected one of the registers 128, 129 by a series of commands fromthe processor 34 which controls the gating of data through buffer 120via bus 125 from register 128 or 129. The response data is written intoregister 128 or 129 as eight bit parallel bytes. The response data istransmitted from the registers 128, 129 over the buses PCMLCA, PCMLCBrespectively by the S/R clock signals in the following manner.

After the last byte of data is stored in the selected register 128, 129,the timing generator 127 will generate an enable signal at apredetermined time over leads A CONTRL or B CONTRL respectively. Theoutputs of gates 141 and 144 are connected to buses PCMLCA and PCMLCBrespectively. The outputs of gates 142 and 143 are "wired-ored" with theleads TSXA and TSXB respectively from the line circuits. The S/R clockis used to shift the response data onto PCMLCA or PCMLCB. Morespecifically, the S/R clock provides pulses at a 4.096 mHz rate, with 42pulses being supplied in a burst during one frame. The first 32 clockpulses of the 42 pulse burst are used to clock the response data fromthe registers 128 or 129 onto bus PCMLCA or PCMLCB.

The last 32 clock pulses of the 42 pulse burst are used to clock thecontrol data from the bus PCMLCA or PCMLCB into the register 128 or 129.During the time when the middle 22 pulses are provided, control data isbeing written into registers 128 or 129 while simultaneously responsedata is being read from the register 128 or 129. This arrangement isprovided because system timing constraints require a 10 bit skew betweenthe receive PCM data bus and the transmit PCM data bus.

Selector 146 is provided for maintenance purposes. The register 147 isalso provided for maintenance purposes and permits the processor 34 todisable either or both of the comparators 131, 133 and to controlselectors 130, 145 and 146.

7.4 SELF TEST (FIGS. 32, 33, 34, 35)

One feature of the control interface 33 is that it includes a self-testmode of operation in which the serial output of one of the shiftregisters 128 or 129 is selected as the serial input data source forboth of the registers 128 and 129. Selectors 145, 146 and 130 providefeedback paths for the registers 128, 129 during the self-test mode. Inthis test mode, the shift registers 128 and 129 are loaded with testdata by the microprocessor and allowed to shift for several frames.Proper operation of the shifting function of the registers 128, 129 andthe address decoder circuit 122 can thus be verified without affectingthe PCM buses PCMLCA, PCMLCB.

A self-test program routine is executed during power-on initializationof the line switch. This self-test routine exercises the circuitry ofthe control interface 33 and produces a go/no-go result.

The test initialized the control interface by loading a code in the modecontrol register 147 which provides a signal on the SELF TEST ENA lineto operate selector 130 and a signal on the FDBK A/B line operatesselector 146 such that a feedback path for register 128 is enabled. Asignal is provided to the timing generator 127 which responds topreventing transmission over the buses PCMLCA, PCMLCB, and by inhibitingthe shifting of the registers 128, 129. The A comparator 131 and Bcomparator 133 are enabled.

Next, A register 128 is loaded with the following test data (inhexadecimal): byte 1=63, byte 2=Cl, byte 3=F8, byte 4=0F. The addresslatch 123 is loaded with F8, i.e., the same as byte 3 on register 128.After the A register and address latch 123 have been loaded, the A and Bregisters 128 and 129 are allowed to shift for eight PCM frames. FIG. 33illustrates the contents of the A and B registers 128, 129 at the startof the self-test and after each frame of shifting. Note that the A and Bregisters 128 and 129 are clocked 42 times each frame as explainedabove. The A and B control registers are 32 bits long. Therefore, thetest data in the A register 128 will be in effect rotated left (leftbeing defined as toward the most significant bit position) a total of 10(42 modulo 32) places after each 42 pulse clock burst. After eightframes of shifting the test data in the A and B registers 128 and 129will have been rotated 16 (42×8 modulo 32) places left of the positionof the test data at the start of the test. Since the address latch 123contains F8 and the A and B address comparators 131 and 133 are enabledan address match should occur with both comparators only during theeighth frame of shifting. If both comparators 131 and 133 do notindicate a match condition during the eighth frame, a failure hasoccurred. If both comparators do indicate a match, the contents of boththe A and B registers 120, 129 are compared with the following expecteddata: byte 1=F8, byte 2=OF, byte 3=63 and byte 4=Cl. Any mismatch of thedata in the A and B registers 128 and 129 with the expected dataindicates a failure.

FIGS. 34 and 35 are a program listing in MSC-48 assembly language codeimplementing the self-test operation for a line group controller.

7.5 AUTOMATIC GAIN SETTING (FIGS. 25, 30, 32, 36, 37 A-C)

The control interface 33 of FIG. 32 includes a voice channel datasampling register 189 which permits the microprocessor to monitorchannel data appearing on one of the PCM buses PCMLCA. This channelsampling feature allows the line group processor to measure and thusprovide automatic gain setting of the line circuits. Register 189 is an8 bit shift register that is serially loaded every frame with dataappearing on a predetermined channel of bus PCMLCA. The PCM SAMPLE SHIFTclock from timing generator 127 clocks the serial data from PCMLCA intothe register 189. Between the 8-pulse clock bursts, data in register 189can be read by the microprocessor sending an appropriate command to thecontrol interface. Note that for clarity, the connection from theaddress decoder 122 to the register 189 has not been shown.

The above described variable gain circuit of FIG. 30 permits adjustmentof the gain of the amplifiers 103 and 105 to a predetermined valuewithout costly hand selection of parts. The analog switches 101, 104respectively control portions of a resistance divider 102, 106 to changethe gain of amplifiers 103, 105, respectively.

In the embodiment shown, the predetermined value for amplifier 103 isset such that the overall loss in the receive path of the line circuitis -0.25dB (decibel). The predetermined value for amplifier 105 is setsuch that the overall gain in the transmit path of the line circuit is0dB. The accuracy of these settings is made to within ±0.1dB. Theincremental difference between adjacent steps of PCM representationsrepresenting the peak required signal levels (of around 0dBm (decibelrelative to a milliwatt test tone)) is approximately 0.4dB for mu-lawencoding. As shown below, it is possible to use the PCM representationsto measure analog signals peaks with an accuracy greater than that ofthe PCM steps.

The frequency for a reference signal used in measuring the gain of thetransmission circuit is chosen as 1.024 kHz sampling rate. By chosingthe frequency of the reference signal in this manner, the referencesignal will be sampled at or near its peak amplitude a predictablenumber of times over a predetermined number of cycles.

For 1.024 kHz signal sampled at an 8 kHz rate, 16 cycles are requiredbefore the PCM representations are repeated. During a 16 cycle period,125 PCM representations are generated.

From mu-law tables it can be determined that the maximum linear level ofa signal that can be encoded is 8159 linear units and, by definition,corresponds to the peak of a signal having an rms (root means square)level of +3.17 dBm. The peak value of the 0 dBm reference signal can becalculated by reducing 8159 by 3.17 dB. In this manner, the peak valueof 0 dBlm signal is determined to be 5664.1785 linear units. From mu-lawtables it can be determined that 566.1785 linear unis lies between level118 which represents 5599 linear units and level 119 which represents5855 linear units. It can be calculated that levels 118 and 119 areapproximately 0.4 dB apart.

If 125 successive PCM samples are monitored and at least one samplecorresponds to level 119 or a higher level, the signal represented bythat group of 125 samples is greater than 0 dBm by more than 0.278 dB.

If all samples in a group of 125 successive PCM samples are less thanlevel 118 then the signal represented is less than 0 dBm by more than0.1 dB. For a 0 dBm signal, the number of samples N out of 125successive PCM sample that are at level 118 either positive and negativepolarity is determined as follows:

The reference signal may be represented as A sin μl. If decision level118 is represented by A' then ##EQU4## where (π/ζ)-Q represents theangular displacement from zero degree where the reference signal reacheslevel 118.

Solving equation (1) for Q yields ##EQU5##

The probability that any one PCM sample in a group of 125 successivesamples will be above level 118 is ##EQU6##

Substituting equation 2 in equation 3 yields ##EQU7##

The number of PCM samples in a group of 125 successive samples whichwill be at level 118 for a 0 dBm signal is

    N=125 P                                                    (5) ps

For a 0 dBm signal

    A=5664

    A'=5599

and N may be calculated as 12.08.

Equations 1-5 may be used to determine the signal levels correspondingto the number of occurrences N of level 118 samples in a successivegroup of 125 PCM samples yielding the results shown in the table below:

                  TABLE                                                           ______________________________________                                        N     A(dBm)     N     A(dBm)    N   A(dBm)                                   ______________________________________                                        1     -.0998      9    -.0447    16  +.0762                                   2     -.0978     10    -.032     17  +.0992                                   3     -.0944     11    -.0173    18  +.1236                                   4     -.0896     12    -.0014    19  +.1484                                   5     -.0834     13    +.0159    20  +.1767                                   6     -.0758     14    +.0346    21  +.2055                                   7     -.0669     15    +.0547    22  +.2357                                   8     -.0565                                                                  ______________________________________                                    

In the illustrative embodiment, if 9 to 16 level 118 samples occurduring 125 successive PCM samples, it is assumed that the samplesrepresent a signal of 0 dBm. From the above table it is seen that thissample range represents 0 dBm over a range of -0.0447 dBlm to +0762 dBm.

Advantageous use of the variable gain circuitry is made to permitautomatic adjustment of the gain setting. Each line circuit shown inFIG. 25 is connected via a test relay 38 to a test bus. The test bus ismultiplied to all the best relays of a line group and may also bemultiplied to all line groups in a line switch module. Alternatively,the serial test buses may be utilized within a line switch module, eachtest bus being multipled to one or more line groups. To set the TX gainof a line circuit, i.e., the gain of amplifier 105, the line switchcontroller will connect an analog milliwatt generator to the proper testbus.

FIG. 36 illustrates the arrangement for setting the transmit gain in aline circuit, the line switch controller 7 will connect a milliwattgenerator 3601 to the proper test bus. The line group processor 34 ofthe line circuit which is to have its transmit gain set will thenactuate the test relay 38 of the line circuit. By means of the sampleregister 189 of the control interface 33 shown in FIG. 32, the linegroup processor will monitor the PCM output of the line circuit. Theline group processor will then incrementally change the gain of thetransmit amplifier 105 in FIG. 30 and balance circuit 41 by selectivelyactuating the analog switches 104 shown in FIG. 30. After eachincremental change in the gain, the line group controller will monitorthe PCM data. The iterative process will continue until the monitoredPCM signals represent a signal of 0 dbm±0.1 dB. The line group processor34 will then release the test relay 38 and signal the line switchcontroller 7 that the transmit gain has been set. The line switchcontroller 7 can then direct a line group processor 34 to set thetransmit gain of other line circuits. To set the receive gain, i.e.,that of amplifier 103 in FIG. 30, the line switch controller will causea digital signal representing a signal 0.25 dB greater than 1 milliwattto be applied on a PCM channel that can be used for testing. One way inwhich this may be done is for the line switch controller 7 to connectthe milliwatt source 3601 providing a 0.25 dB signal to a test bus 3602and direct the line group processor 34 to connect a line circuit, e.g.,line C, which has had its transmit gain adjusted to the desired levelconnected to the test bus 3602. The line circuit C will then provide PCMsignals representative of the milliwatt source on a predeterminedchannel of the PCM bus PCMLCA or PCMLCB. If it is assumed that linecircuit A is to have its receive gain set, then line group processor 34will actuate test relay 38 connecting the output of line circuit A tothe test bus 3603. Another line circuit B which has previously had itstransmit gain set will be connected to the same test bus 3603 byactuating its test relay 38. Thus, the analog output of line circuit Awill be connected to the analog input of the line circuit B. The linecircuit B will be directed by the line group processor 34 to receive thePCM channel which is carrying the digital PCM tone signal from linecircuit C. The PCM output of the line circuit A will be monitored by itsline group processor 34 again utilizing the sample register 189 of thecontrol interface 33.

The line group processor will incrementally change the gain of thereceive amplifier in line circuit A until the output is again 0 dBm±0.1dB. Thus the transmit gain of any circuit can be set to 0±1 db and thereceive gain of any circuit can be set to 0.25 dB±0.1 dB.

FIGS. 37 A, B and C illustrated in flow chart form the program forsetting the transmit and receive gain.

8.0 OTHER LINE CARD ARRANGEMENTS (FIGS. 25, 38, 39 and 40)

The detailed drawing of FIG. 25 may be redrawn in general block diagramform with various leads grouped into buses as shown in FIG. 38. Bus 1701includes PCMLCA, PCMLCB, AILS, and BILS. Bus 1702 includes CLK4LS, FSD,CLKT and CLK2.

The line card control interface 33 as described hereinabove interfacesbetween the line group processor 34 and the PCM buses PCMLCA and PCMLCBfor the interchange of control information over the PCM buses.

In another embodiment of the invention shown in FIG. 39, a separatecontrol bus 1801 is provided and the line group control interface 33 isnot connected to the PCM bus 1701. In this arrangement, controlinformation may be exchanged between the line card control interface 33and a line switch control 7 in the system control 5 over a control bus1801.

In yet another embodiment of the invention shown in FIG. 40, the linegroup processor 34 may directly communicate via bus 1901 with the lineswitch control 7 of the system control 5.

9.0 LINE GROUP PROCESSOR 34 (FIGS. 41, 42 and 43)

The line group processor 34 in the illustrative embodiment comprises anintel 8049 microprocessor which is described in the aforementioned INTELreference at pages 8-32 to 8-35. FIG. 41 illustrates in flow chart formthe main program loop of the software utilized in the illustrativeembodiment. FIG. 42 illustrates the interrupt service routine of FIG. 41in greater detail. FIG 43 illustrates in flow chart form the tablelook-up addresses of various routines called by the command interruptservice routine. Program listings for the line group processor programare shown in the attached appendix.

CALL SET-UP (FIGS. 1, 2, 25)

When a subscriber line (A) goes off-hook, the line switch controller 7of FIG. 2 transmits a common channel command to the system control 5 ofFIG. 1 informing the system control 5 of a new sequence. The systemcontrol 5 responds by transmitting a common channel command to the lineswitch controller 7 assigning a PCM channel A and providing dial tone.

The line switch controller 7 transmit a channel assignment command (32bit control word) to a line group processor 34 of FIG. 25. The linegroup processor 34 receives power to the line circuit for A, assignstransmit and receive channels at the CODEC for A, and updates the statusof line circuit. The system control 5 will then monitor dial pulses fromA.

When the system control 5 determines the called line (B), the systemcontrol 5 will transmit a common channel command to the line switchcontroller 7 of the line switch to which B is terminated. The lineswitch controller 7 will request the busy/idle status of B from the linegroup processor 34 associated with the group of lines including B. Theline group processor 34 responds by transmitting B's busy/idle status tothe line switch controller 7. If B is idle, the line switch controller 7transmit the channel assignment to the line group processor 34. The linegroup processor 34 then enables power to B's line circuit, assigns thetransmit and receive channels to B's CODEC, and updates the line circuitstatus.

9.1 RING AND CALL CONNECT (FIGS. 1, 2 and 25)

The system control 5 sends a common channel command to the second lineswitch controller 7 to connect ringing to B, and turns on a ring-backtone generator to provide ring-back tone to A. If A and B are bothterminated at the same line switch, the system control 5 may alsoindicate an intra-line switch call. The second line switch controller 7sends ring commands to a ring circuit B of FIG. 2 and to the line groupprocessor 34 for B. The line group processor 34 begins a ringingroutine.

When B answers, the system control 5 sends a stop ringing command to theline switch controller 7. The line switch controller 7 in turn sendsring terminate commands to the ring circuit 13 and line group processor34. The line group processor 34 terminates the ring routine.

If the call is an intra-line switch call, the receive channels for A andB are switched upon commands from the line switch controller 7 to theline group processors 34.

9.2 CALLED DISCONNECT (FIGS. 1, 2 and 25)

When A and B go on-hook, the system control 5 sends a common channelcommand to one of the line switch controllers 7 to release the line. Theline switch controller in turn sends a disconnect command to the linegroup processor 34. The line group processor 34 removes power from theline circuit and updates the line circuit status. This procedure isrepeated for the other party.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of our invention as set forth in the objects thereof and inthe accompanying claims.

What is claimed is:
 1. An interface circuit for coupling a line circuitto a control processor and to at least one PCM bus, said interfacecircuit comprises:a first set of terminals coupled to said one PCM bus;a second set of terminals coupled to said control processor andincluding a control terminal, a data terminal and a clock terminal; athird set of terminals coupled to said line circuit and including a pairof PCM terminals coupled to the PCM signal input and output of said linecircuit; first means coupled to said second set of terminals forreceiving command information at said data terminal and for storing saidcommand information in response to said control terminal having apredetermined state; second means coupled to said data terminal andresponsive to said first means for storing control information receivedat said data terminal; third means responsive to said controlinformation stored in said second means for coupling said PCM terminalsto said first set of terminals.
 2. An interface circuit in accordancewith claim 1 comprising:a fourth set of terminals coupled to a secondPCM bus; said third means being responsive to said stored controlinformation for coupling said PCM terminals to said fourth set ofterminals.
 3. An interface circuit in accordance with claim 1 whereinsaid third set of terminals include a first terminal for receivingtiming signals from said line circuits, said timing signals indicating apredetermined PCM channel on said one PCM bus;said interface circuitcomprising circuit means for coupling said first terminal to a secondterminal of said third set of terminals.
 4. An interface circuit inaccordance with claim 2 wherein said third set of terminals includes afirst terminal for receiving timing signals from said line circuit, saidtiming signals indicating a predetermined PCM channel on a selected oneof said first and second PCM buses;said interface circuit comprisingfurther means responsive to said control information for coupling saidfirst terminal to a selected one of second and third terminals of saidthird set of terminals, and wherein said timing signals at said secondterminal determine a PCM channel on said first PCM bus and said timingsignals at said third terminal determines a PCM channel on said secondPCM bus.
 5. An interface circuit in accordance wih claims 3 or 4comprising:a fourth terminal coupled to a supervisory lead; said thirdset of terminals including a fifth terminal coupled to said line circuitfor receiving switch hook information; fifth means responsive to saidcontrol information for coupling said fourth terminal to said fifthterminal whereby said switch hook information is transferred to saidsupervisory lead.
 6. An interface circuit in accordance with claim 5wherein said fifth means is further responsive to said timing signalsuch that said switch hook information is transferred to saidsupervisory lead during said predetermined channel.
 7. An interfacecircuit in accordance wih claim 6 comprising:a sixth terminal coupled toa line carrying PCM clock pulses; sixth means responsive to said storedcontrol information and coupled to said sixth terminal to generatesignals representative of portions of PCM channels; said fifth meansbeing responsive to said sixth means such that said switch hookinformation is transferred to said supervisory lead during apredetermined one of said portions of said predetermined channel.
 8. Aninterface circuit in accordance with claim 7 wherein said sixth means isresponsive to said stored control information to select saidpredetermined one of said portions.
 9. An interface circuit isaccordance with claim 8 comprising: seventh means responsive to saidstored control information for supplying control signals to said linecircuit via said third set of terminals.
 10. An interface circuit forinterfacing a line circuit, a PCM bus and a control processor whereinsaid line circuit includes a pair of terminals connected to a PCM line;said control processor includes a data line, a control line and a clockline; said PCM bus including a bidirectional PCM bus, and a clock lead,said interface circuit comprising:a first set of terminals coupled tosaid PCM bus including a first pair of PCM terminals coupled to saidbidirectional PCM bus and a PCM clock terminal coupled to said clocklead; a second set of terminals coupled to said control processor, saidfirst set of terminals including a data terminal coupled to said dataline, a control terminal coupled to said control line, and a clockterminal coupled to said clock line; a third set of terminals coupled tosaid line circuit including a second pair of PCM terminals coupled tosaid PCM line; first register means coupled to said data, control andclock terminals for receiving and storing first control informationtransmitted by said processor over said data line when said control linehas a predetermined state; second register means coupled to said dataterminal and responsive to said stored first control informationforreceiving and storing second control information transmitted by saidprocessor over said data line; and circuit means responsive to certainof said stored second control information for coupling said first pairof PCM terminals to said second pair of PCM terminals.
 11. An interfacecircuit in accordance with claim 10 wherein said line circuit includes aterminal coupled to a timing signal line, said line circuit providingtiming signals on said timing signal line to indicate a predeterminedone of a plurality of PCM channels;said third set of terminals includinga timing terminal coupled to said timing signal line; said interfacecircuit comprising second circuit means coupled to said timing terminaland responsive to said timing signals for supplying gating signals tosaid circuit means; said circuit means being further responsive to saidgating signals for coupling said first pair of PCM terminals to saidsecond pair of PCM terminals.
 12. An interface circuit in accordancewith claim 11 further comprising third circuit means responsive to saidstored second control information for enabling or inhibiting said timingsignals.
 13. An interface circuit in accordance wih claim 10 whereinsaid second means is further responsive to said stored first controlinformation for transmitting said stored second control information tosaid processor via said data line.
 14. An interface circuit inaccordance with claims 10 or 11 comprising fourth circuit means coupledto said PCM clock terminal and to said circuit means for synchronizingPCM signals from said line circuit to clock signals a said clock lead.15. An interface circuit in accordance with claims 10 and 11 whereinsaid line circuit includes a ring relay terminal coupled to a ringrelay, said interface circuit comprises a first relay terminal coupledto said ring relay terminal; and fifth circuit means coupled to saidring terminal and responsive to said stored second information forsupplying control signals a said ring relay.